Cells use glucose as a main source of energy. Therefore, food is first broken down by the body to glucose prior to being utilized. Glucose is then released from the gut into the blood resulting in a rise in blood glucose levels. In response to this rise in glucose level, pancreatic β-islet cells increase their production and secretion of insulin. Insulin circulates through the blood and acts as a messenger, sending a signal to insulin responsive organs such as the adipose tissue, muscle and liver, to increase their intake of glucose. In this way a rise in blood glucose is accompanied by a subsequent increase in insulin secretion from β-cells. It is the rise in insulin that acts to return blood glucose levels to normal. In healthy individuals blood glucose levels are kept fairly constant. This state of equilibrium, called normoglycemia (normal glucose level) is tightly controlled by insulin.
In diseases such as diabetes this tight regulation of blood glucose level is lost, leading to the increased blood glucose levels observed in diabetics. A state of hyperglycemia (high glucose level) can occur due to an insufficient production of insulin by the pancreatic β-cells and/or through inadequate uptake of glucose by target organs such as muscle, liver and fat. The end result is an increase in blood glucose level. Thus, diabetes can be thought of as the result of two types of impairment impaired insulin secretion from the β-cells and impaired insulin sensitivity by the major insulin responsive organs. This impaired insulin sensitivity, also known as insulin resistance (because the organs are resistant to the effects of insulin), means that more insulin is required in order for the target organs to increase their glucose uptake. Insulin resistance leads to increased pressure on the β-cells because the β-cells need to increase their insulin secretion to compensate for insulin resistance. This is an escalating problem leading first to impaired glucose tolerance and; eventually, complete loss of insulin secretion due to the inability of the pancreas to keep up with the ever-increasing demand for insulin.
Diabetes is a diagnostic term for a group of disorders characterized by abnormal glucose homeostasis resulting in elevated blood glucose. There are many types of diabetes, but the two most common are Type I, also referred to as insulin-dependent diabetes mellitus or IDDM, and Type II, also referred to as non-insulin-dependent diabetes mellitus or NIDDM. Type I diabetes is mainly a disease with a young age of onset, and is due to the destruction of the insulin secreting β-cells in the pancreas by the immune system. In this case the body fails to recognize the pancreatic β-cells as being self and destroys its own cells. With the destruction of the β-cells there is a complete loss of insulin secretion and so affected individuals have an absolute dependency on insulin for survival. Type II diabetes is mainly a disease with a later age of onset, usually after the age of 40, but in recent years it is more common to find younger people being diagnosed with Type II diabetes. It is mainly characterized by insulin resistance and beta cell exhaustion and is often associated with obesity. Type II diabetes is more common than Type I diabetes and accounts for 90-95% of all diabetes cases diagnosed worldwide.
Chronic exposure of tissues to hyperglycemia can result in diverse complications including microvascular problems of neuropathy, retinopathy and nephropathy and the macrovascular complications of stroke, coronary heart disease, and peripheral vascular disease. Inappropriate control of blood glucose level is also a characteristic of diseases other than diabetes such as obesity and Syndrome X. For example, one of the characteristics of Syndrome X is insulin resistance or glucose intolerance. In addition, obesity is characterized by hyperinsulinemia and insulin resistance, a feature shared with NIDDM, hypertension and atherosclerosis. Further, obesity is a major risk factor for NIDDM. The risk of developing NIDDM is tripled in subjects 30% or more overweight, and three-quarters of NIDDM patients are overweight.
Obesity, which is the result of an imbalance between caloric intake and energy expenditure, is highly correlated with insulin resistance and diabetes in experimental animals and humans. However, the molecular mechanisms that are involved in obesity-diabetes syndromes still under investigation. During early development of obesity, increased insulin secretion balances insulin resistance and protects patients from hyperglycemia (Le Stunff, et al., Diabetes 43:696-702 (1989)). However, over time, β cell function deteriorates and non-insulin-dependent diabetes develops in about 20% of obese individuals (Pederson, P., Diab. Metab. Rev. 5:505-509 (1989), and Brancati, F. L., et al., Arch. Intern. Med. 159:957-963 (1999)). Given its high prevalence in modem societies, obesity has thus become the leading risk factor for NIDDM (Hill, J. O., et al., Science 280:1371-1374 (1998)). However, the factors which predispose some patients to alteration of insulin secretion in response to fat accumulation remain unknown. Unfortunately, effective long-term therapies to treat obesity are still not available.
Diabetes afflicts several million people worldwide. In the United States alone, there are more than 18 million diabetics, with 600,000 new cases diagnosed each year. People with diabetes are at higher risk for heart disease, blindness, kidney failure, infection, extremity amputations, and other conditions. It is estimated that the direct medical expenditures and indirect expenditures attributable to diabetes in the United States were $132 billion in 2002. Taken together, diabetes complications are one of the nation's leading causes of death.
Therapies do exist to treat diabetes, such as α-glucosidase inhibitors, biguanides, thiazolidinediones, meglitinides, sulfonylureas and exogenous insulin. However, these therapies have limited effectiveness and are associated with significant safety and tolerability issues such as risk for hypoglycemic episodes, weight gain, gastrointestinal disturbances and anemia. In addition, many of the treatment options require injection or multiple daily dosings which present compliance challenges.
Thus, there exists a need for the identification of an agent which safely and effectively treats a metabolic-related disorder such as, for example, diabetes, atherosclerosis and obesity. The present invention satisfies this need and provides related advantages as well.